Hydroquinone/Quinone Cycle for Reductive Photocatalytic Transformations

Phenanthrenehydroquinones (H₂PQs), generated <i>in situ</i> from phenanthrenequinones under visible light using alcohols as hydrogen sources, act as catalytic organic photoreductants. They deoxygenate efficiently diverse N-heterocyclic <i>N</i>-oxides, including substrates having more negative reduction potentials than the H₂PQ excited-state reduction potential (<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mrow><mml:mi>E</mml:mi></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi>H</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:msup><mml:mrow><mml:mi>P</mml:mi><mml:mi>Q</mml:mi></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mo>·</mml:mo></mml:mrow></mml:msup><mml:mo>/</mml:mo><mml:msub><mml:mrow><mml:mi>H</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:msup><mml:mrow><mml:mi>P</mml:mi><mml:mi>Q</mml:mi></mml:mrow><mml:mrow><mml:mo>*</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:msub></mml:math> = -2.10 V vs SCE), suggesting hydrogen bonding assisted electron transfer. TD-DFT calculations indicate H₂PQs act as both photoacids and photoreductants. This work demonstrates the harnessing of the hydroquinone/quinone cycle in visible-light-driven reductions.